1. Field of the Invention
This invention relates to a method of and apparatus for producing a glass fibre bundle for use in optical communications systems.
2. Description of the Prior Art
As is known, optical fibres for such uses have a light-transmitting glass core with a cladding of a glass of lower refractive index to cause total internal reflection of the light transmitted along the core if it is incident on the interface between the core and cladding.
Optical fibres in the form of single fibre filaments can be used in long distance communications systems, but for medium length applications, such as transmitting signals within an aircraft or a ship, bundles of fibres are preferably used. Such glass fibre bundles are easier to interconnect, in that a reduced accuracy is required in aligning the bundles. Some breakage of individual fibres can also be accepted. Such fibre breakage can occur in use, but they can also occur during the forming of a bundle of glass fibres into a component cable, for example, by extruding a plastics coating or sheath over the fibre bundle.
It has been customary either to marry together fibres to form bundles by re-winding, or to draw a plurality of fibres at one time, sizing them to form a strand or bundle of fibres, which is wound on to a drum. The so-called "pre-form" from which a fibre is drawn is in the form of a clad glass rod, which, for example, can be formed by the process described in our U.K. Pat. No. 1,313,106. Pre-forms in rod form can also be obtained by inserting a glass rod into a tube, and drawing down the composite into a clad glass rod, or by drawing a clad glass rod by a "double crucible" technique. A plurality of such pre-forms can be fed down into a furnace and a mechanical attenuation force applied to each pre-form, so that as it softens it is drawn into a fibre. This can be done, for example, by winding the fibres on to a drum, so that the winding action supplies the mechanical force to attenuate each pre-form into a fibre. Such a winding action produces bundles of fibres with the fibres more or less parallel to one another. The fibres which remain in contact with the winding drum throughout the drawing process are inherently shorter than those which permanently lie on top of other fibers. When unwinding the fibres from the winding drum during further processing (e.g. to provide them with an outer plastics sheath), this slight difference in length reveals itself as a series of loops along the length of the bundle. The application of a size during the fibre drawing process restrains these loops during further processing but does not eliminate them. We believe that these loops contribute to the breakage of the fibres. The greater the number of fibres in the bundle, the greater is the problem created by the loops due to an increase in the difference in length introduced between different fibres in the bundle.